Hairpin vortices in turbulent boundary layers
Hairpin vortices in turbulent boundary layers
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DOI：
10.1088/1742-6596/506/1/012008
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发表时间：
2014-04
期刊：
影响因子：
--
通讯作者：
G. Eitel-Amor;O. Flores;P. Schlatter
中科院分区：
文献类型：
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作者：
G. Eitel-Amor;O. Flores;P. Schlatter
目前的工作解决的问题是否发夹涡是近壁湍流的主要特征，以及它们在过渡过程中发挥的作用。首先，在对应于湍流通道和边界层（Reτ = 590）的平均剪切流中引入单个发夹涡的时间模拟中研究了父子机制。使用从解析模拟计算的涡粘性，湍流背景的影响也被认为是。跟踪下游的旋涡结构，它被发现，二次发夹创建后不久，初始化。此后，所有的旋转结构衰减，而这种效果是强制存在的涡流粘度。在第二种方法中，层流边界层被绊倒，通过插入一个规则的发夹图案，通过定义的体积力表示喷射事件的过渡。这个想法是创造一个合成的湍流边界层主导的发夹状涡。对于Reτ < 250的流动，分析了单个发夹状涡的寿命。时间和空间的模拟表明，再生过程是相当短暂的，可能不会持续一旦湍流背景已经形成。从转捩流场模拟结果可以看出，以前DNS研究中报道的发夹森林是一种外层现象，与近壁湍流的发生无关。
The present work addresses the question whether hairpin vortices are a dominant feature of near-wall turbulence and which role they play during transition. First, the parent-offspring mechanism is investigated in temporal simulations of a single hairpin vortex introduced in a mean shear flow corresponding to turbulent channels and boundary layers up to Reτ = 590. Using an eddy viscosity computed from resolved simulations, the effect of a turbulent background is also considered. Tracking the vortical structure downstream, it is found that secondary hairpins are created shortly after initialization. Thereafter, all rotational structures decay, whereas this effect is enforced in the presence of an eddy viscosity. In a second approach, a laminar boundary layer is tripped to transition by insertion of a regular pattern of hairpins by means of defined volumetric forces representing an ejection event. The idea is to create a synthetic turbulent boundary layer dominated by hairpin-like vortices. The flow for Reτ < 250 is analysed with respect to the lifetime of individual hairpin-like vortices. Both the temporal and spatial simulations demonstrate that the regeneration process is rather short-lived and may not sustain once a turbulent background has formed. From the transitional flow simulations, it is conjectured that the forest of hairpins reported in former DNS studies is an outer layer phenomenon not being connected to the onset of near-wall turbulence.